The first fish I had was a betta. Among his tank decorations were a few little plastic fish on strings, with the string weighted so that the fish stayed in midwater. The betta would attack them, make them run into the glass, wait for them to float back up, and then do it again. He wasn't being aggressive, there was no flaring, he'd just casually drift over and go after it.

Oscars break things by messing with them if you don't give them something entertaining. People who keep them often give them things like ping-pong balls to mess with, or plastic plants to shake apart.

There's a thing called the R2 fish school kit. I used to have it, and I trained a goldfish to push a soccer ball into a goal in exchange for food. I don't think anybody's invented fish toys, but that came pretty close, and the goldfish seemed entertained. I've been considering setting up a 10g and training one of my sunfish to not panic when he's transferred, then I can get that kit again and train the sunfish.